Monitoring and alarm circuitry for a base station transmitting supervising signals to one or more mobile radio stations

ABSTRACT

A base station is provided which supervises the quality of a radio link and including a generator circuit for generating a supervising signal having a desired baseband frequency, a measuring circuit for monitoring the level of the baseband supervising signal, and a transmitting circuit for frequency-modulating a transmitting-frequency carrier of said baseband supervising signal. The measuring circuit comprises an A/D converter for deriving digital sample values from the baseband supervising signal, and a control circuit which monitors the deviation of sample values from that of a digital sample value corresponding to a supervising signal level causing a desired deviation in the output signal of the transmitting circuit on the basis of at least one reference value stored in a memory in connection with the calibration so that the deviation is no more than a predetermined difference value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a base station for a radio system, thebase station supervising the quality of a radio link between the basestation and mobile stations, and includes a generator circuit forgenerating a supervising signal having a desired baseband frequency; ameasuring circuit for monitoring the level of the baseband supervisingsignal; a transmitting circuit for frequency-modulating atransmitting-frequency carrier of the baseband supervising signal andtransmitting it to the mobile radio station, a receiving circuit fordetecting the supervising signal transmitted back from the mobile radiostation and modulating it to a receiving frequency; and a signalprocessing circuit for determining the quality of the used radio link.

2. Description of the Related Art

In the NMT (Nordic Mobile Telephone) mobile telephone system, thequality of the radio link between a base station and a mobile telephoneis supervised by special supervising signals. The base station generatesa supervising signal, e.g. by lowpass-filtering from a clock signalhaving a desired supervising signal frequency. The NMT system utilizesfour supervising signal frequencies with spacings of 30 Hz. The basestation transmits a supervising signal to the mobile telephone, whichreturns the supervising signal immediately to the base station, and thebase station determines the quality of the used radio link, especiallythe signal-to-noise (S/N) ratio, by way of the received supervisingsignal.

When the level of the baseband supervising signal generated at the basestation changes for some reason before transmission, this change inlevel appears as a number of deviation variations in the radio-frequencysignal transmitted from the base station to the mobile telephone andfrequency-modulated by the supervising signal and also affects themeasuring accuracy of the S/N ratio of the radio link. In order toensure that the measured S/N ratio describes the quality of the radiolink sufficiently accurately, the NMT specifications define that thedeviation caused by the supervising signal must not deviate from a setvalue by more than 10%. Today the level of the supervising signal ismonitored by a measuring circuit comprising a rectifier and comparators,for which appropriate threshold voltages corresponding to the lowest andthe highest allowable level of the supervising signal are set by acalibration circuit including adjustable resistors. Such a measuringcircuit is, however, slow and difficult to calibrate, as it requires atleast two adjusters and four adjusting procedures.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a base station havinga more accurate, more reliable, and more easily calibrateable control ofthe supervising signal level.

In the first embodiment of the invention, this is achieved by a basestation provided with a measuring circuit comprising an A/D converterfor deriving digital sample values from the baseband supervising signal.A control circuit is also provided which controls, on the basis of atleast one reference value stored in a memory in connection with thecalibration, a difference between the sample values and a digital samplevalue, corresponding to a supervising signal level causing a desireddeviation in a output signal of the transmitting circuit, is no morethan a predetermined difference value.

In the preferred embodiment of the invention, a microprocessor used asthe control circuit calculates alarm limit values from a reference valuestored in the memory in the calibration of the supervising signal, thereference value being a digital sample value provided by the A/Dconverter from a supervising signal level causing a desired deviation.During the calibration of the level of the supervising signal, themicroprocessor is merely indicated when the measured deviation caused bythe supervising signal is such as desired, whereby the microprocessorstores the sample value obtained by the A/D converter in its memory asthe above-mentioned reference value. The invention thus requires nomanual adjustments. In addition, such a digital comparison is inherentlymore accurate, stable in time and more insensitive to temperaturechanges.

In another embodiment of the invention, the microprocessor comparesdigital samples with reference values stored in the memory in thecalibration of the supervising signal, the reference values beingdigital sample values obtained by the A/D converter from the supervisingsignal levels causing the smallest and the largest allowable deviation.In the calibration of the supervising signal, it is necessary in thisversion to separately determine the supervising signal levelscorresponding to the smallest and the largest allowable deviation, andthe microprocessor then stores the sample values corresponding to thesesignal levels in its memory as alarm limit values. This embodimentrequires more calibration steps than the preceding one; it, however,avoids difficult manual adjustments.

In a further embodiment of the invention, the measuring device measuringdeviation indicates the control circuit directly when the deviation issuch as desired. In still another embodiment, the control circuitcontrols a supervising signal generator on the basis of the deviationinformation it receives from the measuring device in such a way that thedesired deviation is achieved. Thus the whole calibration takes placeautomatically and can be initiated by remote control from outside thebase station, if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by reference tothe accompanying drawings, in which

FIG. 1 shows a block diagram of a base station section required forprocessing supervising signals; and

FIG. 2 shows a measuring circuitry according to the invention formonitoring the level of the supervising signal and required calibrationequipment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The NMT mobile telephone system (Nordic Mobile System) is a so-calledcellular telephone system in which the geographical area covered by thesystem is divided into smaller adjoining geographical areas or cells,each containing one ore more fixed base stations in communication withmobile telephones within the cell. The structure of the NMT system willnot be described in more detail in this connection but the followingpublications are referred to in this respect:

[1] NMT Doc.4.1981 Technical specification for the base stationequipment

[2] NMT Doc. 900-4 Technical specification for the base stationequipment

[3] Addendum to NMT Doc. 900-4 Revised 1988-04-01.

[4] NMT Doc. 900-1.

The above specifications are also referred to regarding signallingbetween the base station and the mobile telephone and especiallyregarding the supervising signal.

Referring to FIG. 1, the generation of the supervising signal at thebase station will be discussed first. The base station comprises asource generating a clock signal, such as a crystal oscillator 16 whichapplies a clock signal f_(c) to an adjustable divider circuit 20. Thedivider circuit 20 divides the clock signal f_(c) by a divisor termdetermined by a frequency setting signal FSET to produce a main clocksignal CLK. The main clock signal CLK is divided by a fixed dividercircuit 2 which produces a supervising-signal-frequency square wavesignal CK and one or more filtering clock signals mCK having preferablya frequency which is the multiple of the frequency of the signal CK. Thesquare wave signal CK is lowpass-filtered by a lowpass-filtering block 1containing a Switched capacitor lowpass filter and a level adjustment.The SC filter is controlled by the clock signal mCK, whereby its cut-offfrequency is adapted automatically to the frequency of the square wavesignals. The output signal of the filtering block 1 is a sinusoidalsupervising signal CS_(T), which during normal operation is appliedthrough a switching unit 14 to a continuous lowpass filter 12, whichfilters the clock signal mCK out of the supervising signal. The outputsignal of the filter 12 is applied to a transmitting unit 15 at the basestation for transmission through a transmitting antenna 18 to a mobileradio station.

The supervising signal CS_(T) is also applied to a level indicator 13which monitors the level of the supervising signal, thus ensuring thatit is in a window defined between predetermined limit values. If thelevel of the supervising signal falls outside this window, the indicator13 generates an alarm signal X₃.

The supervising signal returned from the mobile station and received atthe base station by a receiving antenna 19 and a receiver section 17 isapplied through filters 11 and 3 and the switching unit 14 to a signalprocessing circuitry comprising a mixer 4; filters 5, 6 and 10; signalprocessing blocks 8 and 9; and a level indicator 7, which signalprocessing circuitry indicates the frequency of the supervising signaland measures the S/N ratio.

The structure and operation of the equipment described above aredescribed more closely in Finnish Patent Application 900620.

The present invention relates mainly to the realization of the levelsupervising circuitry 13 and the calibration of the level of thesupervising signal. FIG. 2 shows a block diagram illustrating theprincipal features of the base station equipment required for thepurpose. A generator block 21 illustrates generally the generation ofthe supervising signal and may contain e.g. the components 1, 2, 16 and20 disclosed in connection with FIG. 1.

The output of the generator block 21 is connected to a modulating inputin a frequency modulator 27. The transmitting-frequencyfrequency-modulated output signal of the modulator 27 is applied througha high-frequency amplifier 28 to a transmitting antenna 31. The outputof the generator block 21 is further connected through a rectifier 22 toan analog input in an analog-to-digital converter 23. The operation ofthe A/D converter 23 is controlled by a microprocessor 25 which reads atregular intervals a digital sample value derived by the A/D converter 23from the voltage level of the supervising signal and compares it with areference value stored in an electronic memory 24, the reference valuebeing a digital sample value derived by the A/D converter from thevoltage level of the supervising signal causing a desired deviation inthe output signal of the modulator. The microprocessor 25 generates analarm signal X3 if the sample value derived from the supervising signalduring the operation of the quality control of the radio link deviatesfrom said reference values by more than a predetermined differencevalue. This difference value is preferably ±10% of the reference value.The microprocessor 25 calculates automatically the alarm limits from thestored reference value. The alarm limits may be calculated each time thecomparison is made; alternatively, they can be stored in the memory 24after the first calculation, whereby the sample values can subsequentlybe compared directly with these stored alarm limits.

To calibrate the voltage level of an outgoing supervising signal, amodulation analyzer, such as Hewlett-Packard HP 8903, is connected tothe output of the modulator 27 or the high-frequency amplifier 28 formeasuring a frequency deviation caused by the voltage level of themodulating baseband supervising signal in the outgoing radio-frequencysignal. In the simplest calibration system, the voltage level of thesupervising signal generated by the generator 21 is adjusted manuallyuntil the modulation analyzer 29 indicates that the deviation caused bythe supervising signal is such as desired. Thereafter the performer ofthe calibration applies a control signal from an operating device 30,such as a service monitor, to the microprocessor to indicate that thecalibration of the deviation and the voltage level of the supervisingsignal is completed. On receiving the control signal, the microprocessor25 reads from the A/D converter 23 a digital sample value correspondingto the present voltage level of the supervising signal and stores thissample value in the memory 24 for use as the above-mentioned referencevalue.

In the figure, the operating device 30 is connected to themicroprocessor 25 through another microprocessor 26 which controls theoperation of all speech and control channel units of the base station ina centralized manner. Each channel unit comprises its own supervisingsignal generation, signal processing and level supervising circuitry.

In the other embodiment of the invention, the modulation analyzer 29 maybe connected through the microprocessor 26 to the microprocessor 25, asis shown by a broken line 32. The analyzer 29 may thus directly indicatethe microprocessor of the completion of the calibration of the deviationand the supervising signal level in a desired manner, and thus initiatethe storage procedure described above. The microprocessor 25 may furthercontrol the generator 21 automatically in response to the deviationinformation it receives from the analyzer 29 so as to cause it to changethe voltage level of the supervising signal in such a way that thedesired deviation is achieved. It is thereby possible to carry out thecalibration by remote control from outside the base station.

Alternatively, the microprocessor 25 may monitor the level of thesupervising signal by comparing the digital samples it reads from theA/D converter 23 directly with alarm limit values stored in the memory24 which are also digital sample values derived from the supervisingsignal levels causing the smallest and the largest allowable deviation.The calibration can again be carried out by any one of theabove-described calibration systems, except that the supervising signallevel now first has to be adjusted to the values causing the smallestand the largest allowable deviation before the actual calibration to thenominal value to derive digital sample values from these voltage levelsand to store them in the memory 24 as the above-mentioned alarm limits.This alternative solution thus requires more adjustments than theabove-described preferred embodiment.

The rectifier 22 is not necessary in the measuring circuit but it can beomitted if a sufficiently rapid A/D converter is used.

I claim:
 1. In a base station for a radio system having apparatus formonitoring radio link quality between the base station and at least onemobile radio station by transmitting a test signal from the base stationto the mobile radio station, retransmitting from the mobile radiostation back to the base station a response signal based on the testsignal as received by the mobile radio station, and determiningsignal-to-noise ratio of the response signal as received by the basestation,an apparatus improvement for ensuring that signal strength ofthe test signal remains within pre-selected limits and thereby has agiven range of effect on the frequency range of said test signal and,therefore, on the signal-to-noise ratio of the response signal asreceived by the base station, said apparatus improvement comprising: asignal generator for generating a baseband supervising signal having adesired baseband frequency; a measuring device for monitoring signalstrength of said baseband supervising signal; a frequency modulator forfrequency-modulating said baseband supervising signal on aradio-frequency carrier and for transmitting a modulated test signalthereby produced, for reception by the mobile radio station; a receiverfor detecting the response signal transmitted by the mobile radiostation based on reception at the mobile radio station of said modulatedtest signal; said measuring device comprising an A/D convertereffectively coupled to said signal generator for obtaining time-spaceddigital samples indicative of signal strength of said basebandsupervising signal; a memory device effectively coupled to saidmeasuring device for storing at least one reference correlation betweensignal strength of said baseband supervising signal, and a preselecteddesirable frequency deviation of said modulated test signal; a signalprocessor effectively coupled between said A/D converter and said memorydevice, for comparing said digital samples with said at least onereference correlation; and an alarm device effectively included in saidsignal processor for providing an alarm when comparisons provided bysaid signal processor indicate that signal strength of said basebandsupervising signal correlate to a frequency deviation of said modulatedtest signal which differs by a preselected excessive amount in relationto said at least one reference correlation.
 2. The base stationapparatus improvement of claim 1, further including:said signalprocessor automatically adjusting said signal generator for tending tocause signal strength of said baseband supervising signal to correlatein said signal processor to a frequency deviation of said modulated testsignal which does not differ by as much as said preselected excessiveamount in relation to said at least one reference correlation.
 3. Thebase station apparatus improvement of claim 2, further including:amodulation analyzer effectively coupled to said frequency modulator andto said memory device via said signal processor, for determining, andfor storing in said memory device, each of said at least one referencecorrelation.
 4. The base station apparatus improvement of claim 3,wherein:said modulation analyzer is remotely operable for redetermining,and for updating in said memory device from remotely of said basestation, each of said at least one reference correlation.
 5. In a radiosystem, base station apparatus for monitoring the quality of a radiolink between a base station and one or more mobile radio stations duringnormal operation of the base station,said base station apparatuscomprising: a signal generator for generating a baseband test signalhaving a desired baseband frequency; means for frequency-modulating saidbaseband test signal on a radio-frequency carrier and for transmittingthe resulting signal as a frequency-modulated test signal; a transmitterfor transmitting said frequency-modulated test signal to said one ormore mobile radio stations; an A/D converter, coupled to said signalgenerator, for generating digital sample values based on said basebandtest signal; memory means for storing first and second limit values,corresponding to baseband test signal levels causing the smallest andlargest allowed frequency deviations, respectively, in saidfrequency-modulated test signal as transmitted by said transmitter;means for comparing said digital sample values outputted from said A/Dconverter, with said first and second limit values; and means forgenerating an alarm signal in the base station upon detecting that saiddigital sample values are not between said first and second limitvalues.